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Thermal Soaring How-To Tutorial With On Board Flight Video

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Using on board cameras can reveal aspects of soaring flight not always evident on the ground. I've been shooting lots of inflight video lately using the small but HD quality Mobius action cams. They're light and aerodynamic enough to placed just about anywhere on the airframe with out to much degradation of flight performance. It's useful to see the wing surfaces in action and how the mixes are effecting your flight path. If you think you're flying smooth from your ground position, the cameras will show otherwise, it's amazing to see how much movement happens in the air, and how much the glider gets tossed around by turbulence.

I recently did a instructional narration of a recent flight of my F5J Euphoria with a camera angle that showed the entire wing and tail as the plane thermaled in some stronger conditions. This text expands a bit on what the narration describes and perhaps you can pick some tips that you can apply to your thermal soaring set ups. 

What's first apparent in the video is how little the surfaces are moving in all phases in flight. With the mechanical linkage system optimized for the best resolution and fine tuned with the radio, the plane flies with plenty of control authority yet the surfaces are not moving unnecessarily creating extra drag. A common mistake is to have too much total throw in all surfaces, so when maneuvering, you are always having to correct your over-controlling, resulting in lots of extra control drag. If you have any surface throws reduced down using either limited total throw (end points) or by the use of exponentials, you are throwing away servo resolution and precision. The high accuracy of a digital servo can be wasted through an improper mechanical set up or poor radio programming technique. Getting the wing and fuselage servo system optimized mechanically first to maximize resolution is what I emphasize strongly in all of my building tutorials. 

My Euphoria is essentially pitch neutral, yet flies straight as an arrow at the speed I have it trimmed to via the flight modes. It requires very little elevator input to change attitude, yet pitch is not touchy and the glider gives great feed back through speed changes as the plane flies in different vertical velocities. It can fly long periods free flight at this rear CG position as the pushrod system is slop-free and re-centers perfectly. If I put it in an extended dive with no stick correction it will start to tuck under as it gains speed, but who's going to do that? Add a touch of UP and it will track in a dive until it hits it's terminal velocity or sheds parts, which ever come first.

Pilots are so fearful of the rear CG, like their plane will stall badly to the tail or runaway in pitch if the plane goes too fast or slow. Part of not liking the aft CG feel is that they have to much total pitch throw to start, and a poor mechanical system with a low quality servo. As that CG goes back and the elevator gets more sensitive, they can't stop over-controlling in pitch. To compensate,they then program reduced End Points and/or crank on the EXPO, which deadens the pitch feel at all speeds, the exact opposite of what you want for elevator. 

The solution? Revisit your elevator mechanical set up an make sure you are running the control rod as far inside on the servo horn as possible and make sure there is no double centering. A tight high quality digital elevator servo is also key to improving your elevator feel and makes flying with aft GCS easier. 

On the elevator/stab end, make sure you have the rod on the far end of the horn and make sure the hinges are tight and the surface centers perfectly. Once you have that system optimized, go into the radio and make sure you have the end points turned up enough to maximize the servo throw, and greatly reduce or eliminate the EXPO if you can. You can then start experimenting with more aft CG positions yet still have a nice pitch feel in flight.

Aft CGs do not improve the efficiently of the glider, but aft locations do allow the plane to change speeds more readily as it encounters different areas of lift and sink. This allows you to read the glider from the ground, and this is the key to competition-level thermaling. Reading and reacting to this speed/energy change inflight is what aft CGs are all about. And yes, an aft CG glider turns better, turns tighter without stalling and changes directions faster with less surface deflections.

Later in the video I thermal out inverted. This is actually a good thing to practice, it gets you comfortable with inversion, and it will tell you how nose heavy your plane is better than the dive test. If you are holding a lot of down stick to maintain any sort of airspeed above stall, (no camber, trim for medium cruise speed) then your glider is at least somewhat nose heavy. My Euphoria (and all my planes) require no stick or just a touch of down to maintain inverted flight. Release the stick to neutral when inverted flying level, and watch what the plane does. If it immediately dives down vertical, then nose heavy she is. If it takes a few seconds to dive down, then it is more CG aft. Of course you can trim the elevator to fly inverted hands off with any CG location, but then you would see elevator trim way off (way down trimmed) for any right side up flight speed. 

In the video you can see how the Euphoria is flying locked in, it is maneuvering with very little surface deflection, it maintains its' speed around the thermal turn, and it's not hunting in pitch even though the thermal is rough. My flight modes set the glide speed and camber setting for me, and I fly the bank angle and make small adjustments in pitch attitude when needed. I do fine adjustments of speed with my camber side slider when needed, and for rudder, it is most often moving with the aileron/rdr mix. The trailing edge is working in harmony for roll and camber, I have some elv/camber mix to help with the larger pitch changes. I want the glider to fly its self as much as possible, I'm just there to read to energy changes and suggest more efficient flight paths to explore the location of the strongest lift core.

Can you tell what differential rate on the flaps and ailerons from the video? Nope. Can you tell if I'm using a ail/rudr mix or doing rudder manually? Nope. Can you see the flap to aileron mix for roll? Yes.

Just a few things to think about when you are fretting about your set up. Like I show you in my new Thermal Soaring Master Class, it's the golden triangle of elevator linkage, elevator radio set up, and CG that creates the foundation of a responsive thermal glider. All of the rest of the set ups can be off, but if the elevator response and CG are working together, you can thermal efficiently with minimal pilot work load and easy to read feed back.

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So next time you're out flying, ask yourself these questions:

When was the last time I experimented with the CG position?

Have I optimized the elevator's (or other surfaces) mechanical set up?

How much stick to I have to hold when thermaling?

Do I have the elevator trims set properly for each flight mode?

Do I tend to fly too slow or too fast?

Am I fighting the pitch or roll axis during flight?

Do I use too much EXPO or reduced rates so I can fly smoothly?

Can I tell easily if the glider is in lift or sink?

I'm happy to answer question or give advice. Mail me here:

Paul Naton - Radio Carbon Art


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